1. Field of the Invention:
This invention relates to objectives of variable focal length, and more particularly to such an objective comprising a plurality of lens units of which the first and second counting from the front, i.e. objective side, are of negative and positive powers respectively, and are moved with variation of the amount of air separation therebetween to vary the focal length of the complete objective.
2. Description of the Prior Art:
In the objectives of variable focal length, besides the achievement of good correction of aberrations at a standard setting, it is generally required that the good correction of aberrations be maintained as stable as possible throughout entire range of variation of the focal length. To this end, all the movable lens units must be individually well corrected for spherical aberration, coma and astigmatism. So, each of such lens units is usually constructed with several lens elements.
Recently there have been an increasing demand for reducing the bulk and size of the varifocal objective and extending the range of the focal length. With respect to the former, in the type of varifocal objective comprising two or more lens units of which the first and second counting from the front are respectively negative and positive in the refractive power. The lens units are moved axially in differential relation to each other to vary the focal length of the complete objectives, so far as the paraxial region is concerned, either the power of each lens unit may be strengthened, or the interval between the principal points of the successive two of the lens units may be shortened. With respect to extending the range of variation of the focal length, so far as the paraxial region is concerned, either the power of each lens unit may be strengthened, or the total movement of the zoom lens unit may be elongated. For the paraxial region alone, a common means for achieving both a reduction in the size and an increase in the zoom ratio is found to strengthen the power of each zoom lens unit. For the intermediate and marginal zones, good stability of aberration correction is realized when the number of constituent lenses in each unit is increased in order to strengthen the power of the unit. Another method of strengthening the power of each unit is to increase the surface curvatures of each constituent lens. For this purpose, the minimum acceptable center thickness of the convex lens, or the minimum acceptable air separation of the concave lens surface must be increased. In either case, the overall thickness of each lens unit has to be increased, with the result that it becomes difficult to achieve a much-desired reduction of the physical length of the complete objective. Also, for a given value of the physical length, the longer the overall thickness of each lens unit, the shorter the total movement of the zoom lens unit. A much-desired increase in the zoom ratio becomes difficult to achieve.
In this connection, it should be pointed out that in the above-identified zoom lens unit, positive second lens unit often has the strongest power among all the lens units, thus contributing to a large proportion of the zoom ratio. If a reduction in size is attempted by further strengthening the power of the second lens unit, the necessary number of its constituent lenses increases. Concomitantly, its over all thickness tends to increase objectionably.
The thickness of last lens unit which increases due to the increase in the number of lenses also gives rise to a problem, i.e. if a flippable mirror or a beam splitter is positioned in a space between the rear vertex of the last lens unit and an image plane, a sufficiently large space cannot often be created. Such a vicious circle has placed a limitation on the reduction of the size and the increase of the range of variation of the focal length insofar as any usual homogenous medium lens system is adopted.
As a specific example, suppose a two-unit objective of variable focal length whose negative first and positive second units are moved with decreasing air separations therebetween as when zooming from the wide angle to the telephoto side is designed with an aim to reduce the total length of the entire system from the front vertex to the image plane. Such a design may be achieved by relying on the method of strengthening the power of the second lens unit in combination with shortening of the interval between the principal points of the first and second units. Then, increase of the power of the positive second unit calls for an increase in the necessary number of its constituent lenses to properly correct all aberrations therein itself. This involves an increase of the overall thickness of the second lens unit. Therefore, the principal point interval has to be increased. For this reason, this militates against a decrease in the total length of the entire system.
Such an increase of the overall thickness of the second lens unit has produced another problem. Since it is the last lens unit, for, as a flippable mirror or a prism occupies a space between the rear vertex of the second or last lens unit and the image plane, a somewhat long lens back is required. It is impossible to keep it at the minimum acceptable value.
Meanwhile, using the method of strengthening the power of each lens unit in shortening the total length of the complete objective of variable focal length makes correction of the Petzval sum difficult.
Taking an example of the two-unit objective whose first and second lens units counting from the front are negative and positive in power respectively. The focal length f of the entire system can be expressed by f=f.sub.1 .times..beta..sub.2 where f.sub.1 is the focal length of the negative first lens unit, and .beta..sub.2 is the image magnification. Therefore, even for the same value of image magnification, whichever focal length of the positive second lens unit is shorter, the total length of the entire system can be shortened even more. To use this rule without sacrificing the preservation of the total zooming movements of the positive first and negative second lens units, the second lens unit has to be constructed in the telephoto form with such a power distribution that the principal point is brought farther ahead by increasing the tendency to the second lens unit to be of the telephoto type. So, an attempt to shorten the total length by this method results in a large negative value of the Petzval sum. This implies that the curvature of field is over-corrected.
Further, if the Petzval sum is corrected by lowering the refractive index of the convex lens, or by combining positive and negative lenses of strong power, the spherical aberration is increased very much and very large higher order aberrations are produced. Thus it becomes impossible to achieve good stability of aberration correction throughout the entire range of variation of the focal length. It should be recognized that the reduction of the bulk and size of the objective of variable focal length and the good correction of the Petzval sum are contradictorily related to each other.